[Patent Literature 1] JP 2009-137329 A
A hybrid vehicle which has an engine and motor generator (MG) as a vehicular power source is noted recently from social needs of a low fuel consumption and low exhaust emission. Patent Literature 1 describes a hybrid vehicle in which disposes (i) a motor generator in a power transmission path, which transmits an engine power to a driving wheel, (ii) a first fastening element between the engine and motor generator, and (iii) a second fastening element and a third fastening element between the motor generator and the driving wheel.
An engine-used slip travel is a travel or start-up in a state where the first fastening element is completely fastened and the second and third fastening elements are slipped. In Patent Literature 1, when it is determined that the second or third fastening element is in an overheated state under the engine-used slip travel, a slip amount of one of the second and third fastening elements, whichever is overheated, is reduced while a slip amount of the other is increased. This helps prevent the decline of durability due to the overheating of the fastening element.
In the technology of Patent Literature 1, when the second fastening element or the third fastening element is determined to be in an overheated state, the first fastening element is maintained to be fastened to connect the engine and the motor generator. Thus, neither the second fastening element nor the third fastening element is fastened in a very low speed region providing a transmission gear ratio lower than an engine idling speed. This requires both the second fastening element and the third fastening element to slip. Therefore, overheating of the second fastening element or the third fastening element is not improved. In a worst case, the slip control of the second fastening element or the third fastening element becomes impossible; thereby, the travel only by the motor generator may be unavoidable. This may not achieve a requested driving force.
In addition, either the second fastening element or the third fastening element may be fastened completely. Such a case needs a slip control for a fastening element, which is slipped, with a margin in order to avoid the lock of the transmission gear ratio for fail-safe. This disables securement of the continuity in the requested driving force, possibly degrading drivability.